Molecular Weight of Polymers Crystal Thermal Trans Elastomer Additves

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Molecular weight of Polymers

MOLECULAR WEIGHT OF POLYMERS

Some natural polymers have molecules of same molecular weight (monodisperse)

Synthetic polymers are polydisperse (different polymer molecules have different molecular weights) as they have

distributed molecular weights.

Molecular weights are controlled during the synthesis of polymeric resins.

The properties of polymeric materials are strongly dependent on the molecular weights.

Properties of Polymers depend on the molecular

weight

Almost all properties of polymers have dependence on molecular weight.

Fig. Softening point of epoxy resin increases with increase of molecular weight

Properties of Polymers dependent on the molecular

weight

Fig. Polymer properties vs Polymer size

much lower molecular weight ; poor mechanical property much higher molecular weight ; too tough to process optimum molecular weight ; 15,000 - 100,000 g/mol

Methods for the measurement of molecular weight

of polymers

Mass spectroscopy

Colligative properties measurement

Viscosity measurement

Gel permeation chromatography (GPC)

Gel Permeation Chromatography

Complete molecular weight distribution of polymers can be determined.

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Gel Permeation Chromatograph

GPC can separate complex polymer parts.

Molecular weight distribution

Polymer does not contain molecule of the same size and, therefore does not have a single molecular weight.

Polymer contains large number of molecules- some are big, some small.

Variation in molecular size and weight is known as molecular weight distribution (MWD)

MWD exist in every polymeric system and this determines to a certain extent the general behaviour of polymers.


Average molecular weight of polymers is determined. Two ways of representing molecular weight of polymers:

Number-Average Molecular weight, Weight-Average Molecular weight

Number-average Molecular weight (Mn) Consider a sample of a polydisperse polymer of total weight W in which N=total

number of moles; Ni=number of moles of species i (comprising of the same

size); ni = mole fraction of species i ; Wi = weight of species i ; wi = weight

fraction of species i; Mi = molecular weight of species i; xi = degree of

polymerisation of species i

=

= =

=

=

=1


Number-average Degree of Polymerisation (DPn)

= 0=

Weight-average Molecular weight (Mw)

= =

=

2


Weight-average Degree of Polymerisation (DPn)

= 0=

Example A polymer sample containing 50 mol% of a species of molecular weight

10,000 and 50 mol% of species of molecular weight 20,000.

Mn= (0.5(10,000)+0.5(20,000))/1= 15,000

Mw = [(10,000)2+(20,000)2]/(10,000+20,000) = 17,000

Example Suppose that a polymer consists of 103 chains of M1 = 10

6 g/mol, and 103

chains of M2 = 104 g/mol. Then

Weight average molecular weight is greater than number average molecular weight

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Polydispersity Index

The ratio of weight-average molecular weight to number-average molecular weight is called the dispersion or polydispersity index (I).

It is a measure of width of the molecular-weight distribution curve.

Normal values of I are between 1.5 and 2.5 but may range to 15 or greater.

The higher the value of I , the greater is the spread of the molecular weight distribution of the polymer.

Example

A sample of PVC is composed according to the following fractional

distribution.

(a) Compute Mn, Mw, DPn and DPW

Wt. fraction

0.04 0.23 0.31 0.25 0.13 0.04

Mean mol. Wt. x103 g/mol

7 11 16 23 31 39

Solution

Display data in Table

Wt. Fraction

(wi)

Mean mol.

Wt. (Mi)

wi x Mi wi/Mi

0.04 7,000 280 0.57 x 10-5

0.23 11,000 2,530 2.09 x 10-5

0.31 16,000 4,960 1.94 x 10-5

0.25 23,000 5,750 1.09 x 10-5

0.13 31,000 4,030 0.42 x 10-5

0.04 39,000 1,560 0.10 x 10-5

Total 19,110 6.21 x 10-5

Solution Using equation

Using equation

Number of molecules per gram = (6.21 x 10-5) x (6.02 x 1023)

= 3.74 x 1019 molecules per gram

=1

=1

6.21 105= 16,100 g/mole

= = 19,110 g/mole

= 0=16,100 /

62.5 /= 258

= 0=19,110 /

62.5 /= 306

1 mer weight of vinyl chloride (C2H3Cl) = 62.5 g/mer

Effect of molecular weight and molecular weight

distribution on Physical and Mechanical properties

High molecular weight increases

Tensile strength

Impact toughness

Creep resistance

Melting temperature

Entanglement of chains


distribution on Mechanical properties

More entangled the molecules, the more tensile force would be required to cause them slide.

Narrow MWD results in higher strength than broad MWD. Impact toughness increases with molecular weight (long chains would

transmit energy along the chain and share over more atoms, resulting

in dissipation of energy through vibrations, minor translations, and

heat).

Entanglement also allow some of the energy transport to other molecules, resulting in decrease in concentration of energy.

Broad MWD result in poor ability to transmit the energy between the chains.

Higher MW increases mechanical properties and broader MWD will decrease the properties.

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distribution on Melting point

More entangled the molecules, the more input energy required to get free movement and melting point increases.

High molecular weight enhances materials thermal stability and makes them suitable for high temperature applications.

Low molecular weight generally reduces melting point and improve ease of processing.

Crosslinks increase molecular weight, resulting in increased melting point and mechanical properties.

Melting

point

Mechanical

properties


distribution on processing

For some types of processes, a narrow MWD is preferred while in

others broad MWD give better processing.

Narrow MWD means that material will melt over a narrow range of temperature (same size molecules will require same amount of energy

to cause them to move freely)

Narrow MWD is suitable for injection moulding which depend on freezing of molten polymer.

Extrusion requires broad MWD. High melt strength (measure of ability of molten material to be

shaped) is achieved when MWD is broad. The small molecules melt

first and lubricate the entire mass giving some ease of sliding to large

molecules.

Broad MWD lowers effective melting temperature whereas the large polymers give strength to the melt because of their residual

entanglement.

Bimodal MWD provide ease of processing (UHMWPE).

Melt Index

A parameter used to describe polymer.

Flow characteristics of the polymer are strongly dependent on MW and MWD.

A melt index measurement is a very easy method of assessing the MW of polymers

(commonly used in industry).

Definition: It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a

specific diameter and length by a pressure applied via

prescribed alternative gravimetric weights for

alternative prescribed temperatures.

High melt index indicates a low molecular weight and vice versa. Melt index gives information about molecular weight but not MWD. Ease of melting of polymer can also be determined. High melt index numbers (ease of melting, lower energy input and easy

processing.

Length of Polymer chain

= ( ) .

For a polyethene molecule with Dp = 1000, the molecule

would have a maximum length by:

= . .

= 2,450

Length of Polymer chain

Anisotropic nature of polymer chains

Conformation

The change in shape of a given molecule due to torsion about single () bonds referred to as change in conformation.

A polymer molecule can take many different shapes due to its degree of freedom for torsion about bonds.

Rapid change in conformation

is responsible for the sudden

extension of rubber and high

flexibility of polymers.

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Shape (Steric) Effects

Effects of the shape or size of the atoms on the properties

of the polymer are called Steric effects.

Steric effects determine properties of polymers

Shape and size of the pendant groups have major effect on some of the important properties of polymers

Large pendant groups, crystallinity decreases (strength and thermal properties are lower for such polymers)

But

Pendant groups increase strength because of interference of the bulky groups of chains.


Bulky groups hinder the movement of the chains

Hindered movement increase mechanical and thermal properties similar to intermolecular forces or

entanglements

Influence of Bulky side groups (steric hinderance)

Crystallinity decreases Restrict the movement

of chains

Strength increases, elongation decreases and

thermal properties increases

Strength decreases, elongation increases and

thermal properties decreases

Net effect determines properties of polymers


Aromatic groups are very bulky and cause the steric hindrance effects

Some pendant groups are flexible such as aliphatic groups

Aliphatic groups do not inhibit translational, rotational or flexing movement.

Branched chain polyethene (LDPE) is very flexible material due to aliphatic side branches despite reduction

in crystallinity.

Aromatic pendant group decreases crystallinity,

increases strength and decreases elongation

whereas Aliphatic pendant groups decreases

crystallinity and strength but do not decrease

flexibility


Pendant groups (aromatic groups) in backbone affects physical properties considerably.

A stiff backbone results in high strength, impact toughness and higher thermal properties.

Aramid fibres (Kevlar and Nomex) have aromatic groups in backbone.

Aliphatic chains are more flexible.

Effect of Substituent group/bulky groups/pendant groups

Bond rotation

become difficult

Bulky side groups

n

CH2 CH2

CH CH2

CH3

n

CH2 CHn

Strength, rigidity, modulus

Flexibility, elongation, crystallinity

Decreases Increases

CH2 C

CH3

C O

O CH3

PE

PP

PS

PMMA

Classification of polymers on the basis of steric effect

Steric structure of chains

High packing ( increased crystallinity),

decreased elongation, high strength, High melting points,

good resistance against solvents and chemcials

Thermoplastics

Small branches decreases viscosity and

Improves workability in melt Long branches increase viscosity,

Increase ductility and low strength

3-D polymers, Increased strength

and rigidity, Highly brittle, No ductility,

Donot melt instead Degrade and char

Thermoset

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Geometrical Isomerism/Chain configurations

Isoprene exhibit geometrical Isomerism

Cis-Isoprene Trans-Isoprene

Natural rubber is cis-polyisoprene

(has elsotmeric nature)

Trans-polyisoprene (Gutta Percha)

has rigid nature

Head to Tail and Head to Head or Tail to Tail configurations

Polymer Crystallinity Packing of molecular chains

to produce ordered structure

Polymers are:

Semi-crystalline

Amorphous

LDPE = 50 % crystallinity

HDPE = 75 % crystallinity

Any chain disorder or

misalignment (twisting,

kinking, coiling) result

in amorphous regions

Morphology of polymers Structure in the solid state,

size and shape of crystal and

crystal aggregates

Polymers have small

crystalline regions which are

dispersed in amorphous

regions

TEM of single crystal of polyethene

having thickness 10-20 nm and 10 m

long

Multilayered structure

Fig. Schematic drawing of single crystal with regular chain folding

Fringed Micelle Model of Semi crystalline polymer

Chain folded model

Crystalline region

Amorphous region

Morphology of crystallites from Polymer melts

Spherulites lead to grain

boundaries

Spherulite structure in natural rubber

Morphology of crystallites from Polymer melts

Spherulites are similar to grain boundaries in polycrystalline materials.

PE,PP,PVC,PTFE and Nylon form spherulitic structure when they

crystallise from the melt.

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Kinetics of Crystallization

Formation of nuclei

Growth of crystallites ( spheurlites formation in all direction depending on

external conditions, High supercooling

creates many small crystallites while

crystallization at higher temperature

produces fewer spherulites)

Factors controlling degree of crystallinity

1. Rate of crystallization

2. Controlling chemistry

3. Side groups

4. Tacticity

5. Branching

6. Cross-linking

Pre-deformation by Drawing

Heat treating

Increasing crystallinity of polymers after solidification

Crystallinity Increases Tensile strength, Melting point,

Decreases ductility, Increase resistance

to dissolution

Determination of crystallinity

By measuring specific

volume/densities

DSC

NMR

XRD

c= density of 100 % crystalline material

a= density of 100 % amorphous material

s= density of unknown sample

Thermal Transitions in Polymers

Heating polymer Polymer chains move internally to absorb energy input

Molecular twisting, vibrating, stretching, translation and other movement

Heat distortion

temperature, HDT

Glass transition temperature,Tg

Melting

point, Tm

Decomposition

temperature, Td

Increasing

temperature

Hard

Stiff

Glass like

Limited atomic

movements

small volume

increases

Moderately hard and stiff

Creep

Slightly higher atomic

movements

Small volume

increases

Limit for structural

applications

Pliable, leathery

Larger, longer-range

and

coordinated

movements

Liquid

Entire polymer

molecules

move

independently

Degradation

Chain breakage

Gas release

Char formation

Dramatic and non-reversible

change in

properties

Color change

Heat Distortion Temperature, HDT

weight

Height gauge

Stirrer

Heat transfer

fluid sample

Thermometer

ASTM, D648, Deflection under load test to determine heat distortion temperature

Sample dimensions

= 5 x x inch

Maximum structural use temperature ,

especially for any application in which

the part will be loaded mechanically

Glass Transition Temperature The thermal transition that occurs when the

polymer molecule begins to make

coordinated long-range movements is called

glass transition temperature (Tg). Polymer

becomes pliable and leather like.

HDT is not formal thermal transition HDT is easier to determine than Tg

Measurement of Glass transition temperature

1.By measuring changes in specific

volume with temperature

Chain

mobility

freezes

Chain

mobility

increases

Tg

Increase in

temperature

Tg is a temperature at which

frozen free volume of 2.5 %

appears and stays constant at

low temperature

2. Differential Scanning

Calorimetery

DSC monitors heat effects associated with phase transitions and

chemical reactions as a function of temperature

In DSC difference in heat flow to the sample and a reference at

same temperature is recorded as a function of temperature. The

difference in heat flow can be positive or negative.

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DSC curve

The integral under the DSC

peak , above the base line,

gives the total enthalpy

change for the phase

transition

Determination of Crystallinity by

using DSC curve

100material same of sample ecrystallin totally theofEnthalpy

Sample theofEnthalpy % Crystallinity =

Multiple Transitions

Polycarbonate Gamma

transition ,

T= -100 oC

Gamma transitions

took place due to the

presence of certain

side groups ( may

increase toughness

of polymer)

Factors effecting glass transition temperature

Bulky side

groups

Polar side

groups

Double chain bond or

aromatic groups

Crystallinity

Molecular weight

Branching and

crosslinks

LDPE Tg = -110 oC

HDPE Tg = -90 oC

Tg = 0.5-0.8 Tm (K)

Tg of different vinyl polymers

Melting of polymers Melting is simply the process of a polymer chains

gaining sufficient energy to move independently

HDT Tg

Tm Td

Hard, stiff Leathery Liquid

Thermoplastic (Amorphous)

Thermoplastic (crystalline)

HDT

Tm

Temperature Char

Tm Td

Hard , stiff Liquid Char

Thermosets

HDT Tg Td

Hard, stiff Semi-rigid

General behaviour of thermoplastics and thermosets

Decomposition temperature, Td When input energy has localized in the bond and equals the bond energy (breaking of

covalent bonds occur), the corresponding

temperature is called decomposition

temperature. Thermoplastics decomposition occur

in liquid state release gas and may

form crosslinks.

Thermosets decomposition occur in

solid state forming char ( by product

of gases are often released and the

polymer may begin to change color,

often yellowing or blackening.

Measurement of decomposition temperature DSC or DMTA

TGA ( easier and most commonly used

method) TGA Sample is progressively heated

and changes in the weight of the

material are recorded

Processing Temperature this is a temperature at which plastic

material can be conveniently

molded TP = Tm + 50 to 100 oC

Non-thermal energy inputs energy input from mechanical source

or from any other sources (sound ,

light, x-ray)

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Thermogravimetric analysis Onset of degradation

TGA curve of a polymer

Differential thermal analysis (DTA)

Schematic illustration of DTA cell

Changes in sample which lead

to absorption or evolution of

heat can be detected relative

to the inert atmosphere

exothermic

DTA curves

Diffusion in Polymers

Permeability and absorption characteristics related to the degree to which foreign substances diffuse into the material.

Penetration of these foreign substances can lead to swelling and chemical reactions with the polymer molecules, which often leads to

degradation of material and loss of physical properties.

Diffusion rates are greater through amorphous substances than crystalline.

Size of foreign molecule affect the diffusion process. Small and inert molecules diffuse faster.

Diffusion in polymers according to Ficks law is defined as

J = diffusion flux of a gas through the membrane (cm3 STP)

PM = Permeability coefficient = DS, (D= diffusion coefficient and S= solubility of

diffusing species in polymer

x = Membrane thickness P = Difference in pressure of gas across the membrane

Diffusion in Polymers

Applications requiring low permeability rates:

Automobile Tires and inner tubes, beverage and food packaging.

Application requiring high permeability rates:

Polymer membranes used as filter to separate one chemical from

another (desalination of water)

Polymer electrolyte fuel cells.

Example Problem Mechanical and Thermomechanical Behaviour of Polymers

Stress-strain behaviour

Brittle (thermosets and some thermoplastics)

Ductile (thermoplastics)

Totally elastic (elastomers)

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Deformation of Semicrystalline Polymers

Mechanism of Elastic deformation

Elongation of the chain molecules from their stable conformation in the direction of the applied stress by bending and stretching of the

strong chain covalent bonds.

Some displacement of adjacent molecules resisted by intermolecular forces.

Elastic modulus is combination of the moduli of crystalline and amorphous phases.

Elastic deformation in polymers occur at low stress level.

Mechanical and Thermomechanical Behaviour of Polymers

Effect of temperature on mechanical properties

Decrease in modulus

Reduction in tensile strength

Increase in ductility

Effect of rate of deformation

Slower deformation result in decrease in modulus, decrease in strength and higher elongation.


Stages of Elastic deformation

Two adjacent chain-folded

lamellae and interlamellar

amorphous material before

deformation

Elongation of amorphous tie

chains during the first stage

of deformation

Increase in lamellar crystallite

thickness (which is reversible)

due to bending and stretching of

chain in the crystallite


Mechanism of Plastic deformation

Interactions between lamellar and intervening amorphous regions in response to applied tensile load.

Two adjacent chain-folded

lamellae and interlamellar

amorphous material before

deformation

Tilting of lamellar chain folds

during second stage

Separation of crystalline

block segment

Orientation of block segments and tie chains with the tensile

axis in the final deformation stage

Heating specimen at

some arbitrary stage of

deformation will allow

material to regain

spherulitic structure and

also shrink (depends on

the annealing

temperature and also the

degree of deformation)

Deformation of Semicrystalline Polymers Deformation of Semicrystalline Polymers

Macroscopic deformations of polymers

Methods of increasing strength of polymers:

Molecular weight , Degree of Crystallinity, predeformation by drawing, heat treating

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Elastomers

Criteria for polymer to be called as Elastomers Rubber like elasticity (ability to bounce back) Ability to undergo large deformations and then elastically spring back to

original form

Ability to spring back arises due to light cross-linking between the molecular chains.

Highly amorphous and composed of cross-linked chains that are highly twisted, coiled and kinked.

Chain bond rotation must be relatively free for the coiled chains to readily respond to an applied force.

The onset of plastic deformation must be delayed. Elastomer must be above glass transition temperature.

Properties: Low elastic modulus and vary with increase of strain. Non-

linear stress-strain curves.

Deformation of Elastomers

Highly coiled, twisted and

cross-linked chains

Chains straighten up upon application of

tensile force

Driving force for elastic deformations

Entropy (measure of degree of disorder within the system) Elastomer experiences rise in temperature and modulus increases with

increase of temperature unlike to metals.

Ordered chain

(low entropy)

Disordered chain

(High entropy)

Increase in Temperature

Elastomers

Vulcanisation The process of crosslinking in elastomers is called vulcanisation.

Useful elastomer is obtained by reacting 1 to 5 parts of sulphur with 100 parts of rubber (1 crosslink for every 10 to 20 repeat units)

Cross linking of natural rubber (cis-isoprene)

Comparison between Vulcanised and Unvulcanised rubber

Unvulcanised rubber with very few cross links is soft, tacky and has poor resistance to abrasion.

Modulus of elasticity, tensile strength and resistance to degradation by oxidation are enhanced by vulcanisation.

Types of Elastomers

Aliphatic Thermoset elastomers

These are the most common elastomers. These have a double bond after polymerization has occurred. These are noncrystalline. These are highly flexible.

Natural rubber (extracted from tree)

Types of Elastomers

Synthetic Polyisoprene or Isoprene rubbery (IR)

Produced to meet the shortage of natural rubber during WW. Mixture of both cis and trans molecular form. Used for tire for light weight vehicles such as bicylces and early automobiles. Natural rubber is used extensively because of its low cost.

Butadiene rubber (BR)

No cis or trans isomers Lower mechanical strength because of no pendant methyl group but also more

flexibility

Lower cost (all synthetic from cheap monomer) Improvement of low-temp flexibility Compatibility with other polymer materials Poor oil resistance and sensitivity to oxidation and UV radiation.

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Types of Elastomers

Oil resistant elastomers

Nitrile Butadiene rubber (NBR)

Copolymerization of butadiene and acrylonitrile More expensive than SBR or BR Excellent oil and oxidation resistance. Has good abrasion resistance but poor low temperature elasticity. Typical applications include: oil and fuel lines, gaskets, seals, conveyor belts and

coatings for printer rolls.

CRChloroprene rubber (neoprene)

Thermal stability Non-flammable Applications : fuel hoses, boots, shoe soles, and coatings for fabrics where oil

resistance and nonflammability are important.

Example Problem

How much sulphur must be added to 100 g of polyisoprene rubber to cross

link 5 percent of the mers? Assume all available sulphur is used and that

only one sulphur atom is involved in each cross linking bond.

MW (polyisoprene) of mer unit = 68 g/mol

Mole of polyisoprene = 100 g/68 g/mol = 1.47 mol of polyisoprene

For 100 % cross-linking with sulphur we need 1.45 mol of S or

1.47 mol x 32 g/mol = 47 g of sulphur

For cross-linking 5 percent of the bonds we need only

0.05 x 47 g = 2.35 g of S

Types of Elastomers

Thermoplastic elastomers

Not cross linked. Noncrystalline. Aliphatic. Ease of processing and recycle able More temperature sensitive. Not well developed.

Ethylene Propylene Monomer rubber (EPR)

Amorphous Improved oxidation resistance and improved acid and alkali resistance over

natural rubber but have poor compatibility with other rubbers, poor creep

resistance, relatively poor resilience and poor resistance to hydrocarbon solvents.

Applications: bumpers, hoses, seals, mats, wire insulation , appliance parts , gaskets and coated fabrics.

Types of Elastomers

Thermoplastic elastomers

Copolymers such as SBR and SIS are thermoplastic elastomers

Types of Elastomers

Silicones

Three forms--- oil, elastomers, moulding compound, sealants, adhesives Oils are used as mould releases, coolants, lubricants, hydraulic fluids, etc. Silicone elastomers have high molecular weight and crosslinked chains. Much higher molecular weights than former are called silicone moulding

compounds.

Advantages of silicone elastomers and moulding compounds: Low surface tension, nonionic/nonpolar characteristics , hydrophobicity, high thermal stability

(250 C) , oxidation resistance, high degree of flexibility at low temperatures, room

temperature curing (RTV),etc.

Biomedical implants and tubing ,etc make excellent use of its superior biocompatiblity.

Poly dimethyl siloxane

Different forms of degradation of polymers

Degradation

(An Irreversible process leading to a significant change in

the structure of a material, typically characterized by a

loss of properties and/or fragmentations)

Stabilization

(The protection of polymeric materials

from which lead to deterioration of

properties)

Photo-degradation

(Degradation preceded by light (UV) Photo-stabilization

Screening radiation Absorption of radiation Radical Scavenging

Bio-degradation

(Degradation processes in which at least

one step is mediated by biological agents) Bio-stabilization

Chemical inertness Coating of anti-microbial agents

Thermal Degradation (Degradation caused by heat and temperature)

Thermal Stabilization Flame Retardancy Introduction of thermal stabilizers

Ultrasonic Degradation (Degradation caused by Ultrasonic sounds)

High Energy Degradation (Degradation caused by high energy radiations like X-ray, ,, rays)

High Energy Introduction of radiation protectors

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Polymer Additives/Compounding ingredients

Functions of Additives

Facilitate processing without degradation or decomposition To cater the requirements of a specific application.

Desirable Properties of Additives

Additives can be solids, liquids or rubbery materials.

Stable under the processing conditions and also under the conditions of their applications.

Efficient, i.e, small quantity should do the required job. Compatible i.e, non-bleeding Non-toxic and not impart any taste and odour to the polymer and should not

negatively effect the inherent properties of the polymer

Cheap.

Types of Additives

Fillers

Functions:

Improvement of physical properties. Making the material cost less.

Fillers may be organic or inorganic, mineral, natural or synthetic in nature. They may

be particulates, fibrous, resinous or rubbery materials.

Examples, addition of saw dust or wood flour in phenol formaldehyde.

Reinforcing Fillers

Improve the mechanical properties of the polymer.

Can be called as composites.

Types of Additives

Plasticisers and softeners

Functions:

Improvement in ductility/flexibility Reduce glass transition temperature.

External Plasticisation

Addition of glass micro-spheres bring about plasticisation effect by acting as a

spacer between the molecules of the polymer.

Internal Plasticisation

Incorporation of polymer molecules in the polymer so that they become part of the polymer structure.

Addition of various types of esters can bring about internal plasticisation effect. Examples trioctyl phosphate, phthalic acid ester , etc.

Types of Additives

How Plasticisers work?

Types of Additives

Anti-aging Agents

Functions:

To minimise the structural changes (chain rupture, cross-linking, chromophoric groups, polar groups) occur due to chemical reactions such as oxidation, ozone

effect, UV light attack ,etc.

To increase the service life of polymers

Antioxidants (thiobisphenol,alkylphenol), UV-light abosrobers (hydroxybezophenone)

etc are added.

Types of Additives

Antistatic agents

Functions:

To stop the build up of electrostatic charges on the surface of polymeric materials which can lead to fire and dust catching.

Compounds such as fatty amides, quaternary ammonium compounds, glycol

esters and sulphates are very common agents. They are usually incompatible

with the polymer due to which they slowly migrate to the surface and give a

protective layer.

Polyethylene glycol is a very widely used antistatic agent for PVC conveyor belting.

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Types of Additives

Blowing agent

Functions:

To produce cellular polymers (foams,sponges)

Blowing agent at the manufacturing temperature turn to a gaseous form to pervade

the mass of the polymer.

Common blowing agents are azo-dicarbonamide (for PVC and polyolefins)

which decomposes at 190-230 C giving off N2, CO and CO2 gases.

Types of Additives

Lubricants and flow promoters

Functions:

To prevent sticking of the polymer to the metal walls.

External lubricants have low solubility and carbon chain length C22-C32. Examples

are stearic acid, myristic acid, paraffin wax, etc.

To improve the flow of the polymer melt.

Internal lubricants remain within the mass of the polymer reduce the cohesive forces

of the molecular interfaces and thus improve the flow of polymer melt. Examples are

stearyl alcohol, metal stearates, monoglycerin esters, etc.

To reduce friction in the final products.

MoS2 and graphite are added in small quantities (1-2 %) to reduce friction in

applications such as nylon gears and bearings, etc.

Types of Additives

Colourants

Functions:

To give aesthetic appeal. To give a means of identification.

Four methods of imparting colours to Polymers

Coating the surface Dyeing the surface Incorporation of the materials which would provide chromophoric groups in the

polymer

Colourant fall into two groups- dyes and Pigments

Dyes give colour by dissolving in the polymer (much better against fading but

sensitive to light)

Pigments disperse throughout the mass of the polymer (tend to migrate more)

Types of Additives

UV-degradants

Functions:

To facilitate the disposal of the polymers after they have been used.

Photodegradants

On absorbing light, the heat generates a highly reactive chemical intermediate which

destroys the polymer.

Iron di-thio-carbamate is an example of such an agent. The photo activator reduces

the molecular weight of the polymer below 9,000 at which polymer becomes

biodegradable.

Polymer P* (reactive intermediate) Polymer of lower mol. wt UV-radiation

Liquid Crystal Polymers

LC polymers have liquid crystalline state LCP have ability to be aligned in highly ordered configuration As solids form domain structures having characteristics intermolecular

spacing.

Partially crystalline aromatic polyesters based on p-hydroxybenzoic acid and related monomers.

LCP have outstanding mechanical properties at high temperatures, excellent chemical resistance, inherent flame retardancy and good

weatherability.

Liquid Crystal Polymers

LCP are used in LCDs on digital watches, televisions, monitors, etc.

Cholestric LCPs are used for LCDs as they are liquid at room temperature, transparent and optically anisotropic.

LCPs are sandwiched tween two glass sheets. The outer surface is coated with conductive film. The characters are etched into the film on the

side to be viewed. Application of voltage will cause disruption in the

orientation of the LCPs molecules resulting in darkening of the LCP

material and in the formation of a visible character.

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Fracture in Polymers

Thermoset have brittle fracture

Thermoplastic experience both ductile and brittle fracture

Brittle fracture occurs due to

Reduction in temperature Increase in strain rate Presence of notch in the sample Any modification in polymer which increases glass transition temperature


Crazing

Some thermoplastic experience crazing Formation of microvoids, growth and coalescence

results in crack formation


Thermoset have brittle fracture

Thermoplastic experience both ductile and brittle fracture

Brittle fracture occurs due to

Reduction in temperature Increase in strain rate Presence of notch in the sample Any modification in polymer which increases glass transition temperature

Documents

Molecular Weight of Polymers Crystal Thermal Trans Elastomer Additves